In typical exhaust sampling systems, such as constant volume samplers (CVS), engine exhaust is diluted with a make-up gas, or diluent, and a sample of the diluted exhaust is proportionally extracted and stored in one or more sample bags. Depending upon the engine size, drive cycle and ambient conditions, the CVS total flow rate, which includes both the make-up gas and engine exhaust, is selected to ensure the diluted exhaust sample does not condense water when stored in the bags.
In addition to determining an appropriate CVS total flow rate, some systems fill the sample bags with a gas, such as a pre-fill gas, to avoid bag condensation. Some other systems apply heat to the sample bags such that the temperature of the sample is maintained above the dew point.